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Environmental Health and Safety (EHS) of Gas Boxes/Cabinets

Joseph B. Barsky, MS, CIH and Russ McDonald

October 27, 2011

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Objectives

• Regulations and Terminology

• Best Engineering Practices

• Safety Controls are not "Customer Options"

• EHS considerations when buying used equipment

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Regulations and Terminology

• Regulations and Terminology

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Definitions

Gas cabinet: An exhausted enclosure used to contain cylinders of process gases that contains leaks without risk of affecting people. A well-designed gas cabinet can also accommodate panel-mounted manifolds and gas handling systems that provide precise control over operating parameters and total purging of all equipment and lines. They protect personnel with a safe, efficient and cost-effective means to organize gas distribution.

Gas box: A subsystem of the factory gas delivery system located at point-of-use, typically internal to a semiconductor process tool, and utilized to control delivery of process gases into the processing chamber. Also referred to as gas interface box and gas jungle.

Mass flow controller (MFC): A self-contained device (consisting of a transducer, control valve, and control and signal-processing electronics) commonly used in the semiconductor industry to measure and regulate the mass flow of gas.

Mass flow meter (MFM): A self-contained device, consisting of a mass flow transducer and signal-processing electronics, commonly used in the semiconductor industry to measure the mass flow of gas.

Pressure transducer: A device commonly used in the semiconductor industry to measure gas pressure. Typically consisting of a sensor and signal-processing electronics, this device allows for remote indication of gas pressure.

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Definitions

Purifier: An in-line device used for the removal of homogeneous impurities from gases, typically consisting of a packed-bed of active solids contained in a stainless steel housing. The active purification media may remove impurities such as moisture, oxygen, CO, CO2, hydrocarbons, hydrogen, or nitrogen from specific gases using a variety of mechanisms. Point-of-use purifiers often contain a particle filter within the same housing.

Regulator: A valve designed to reduce a high incoming pressure (for example, from a cylinder) to a lower outlet pressure by automatically opening to allow flow until a desired, preset pressure on the outlet side is reached, then automatically throttling closed to stop further pressure increase.

Valve: (1) A device that controls the flow or pressure of a gas. Valve functions can include shutoff, metering, backflow prevention, and pressure relief. (2) Any component designed to provide positive shutoff of fluid media with the capability of being externally operated. Newer systems should have lockable valves to meet Lockout/Tagout requirements.

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Typical Gas Systems

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Regulations and Standards

Fire and Storage requirements may differ from location to location.

• California Fire Code, Chapter 27

• International Fire Code

• Uniform Building Code

• Uniform Fire Code

• Cal/OSHA / Federal OSHA

• NFPA 704

• SEMI

• CGA

• ANSI

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Some Pertinent SEMI Standards

• SEMI F1- Specification for Leak Integrity of High Purity Gas Piping Systems and Components

• SEMI F3- Guide for Welding Stainless Steel Tubing for Semiconductor Manufacturing Applications.

• SEMI F4- Specification for Pneumatically Actuated Cylinder Valves

• SEMI F5- Guide for Gaseous Effluent Handling

• SEMI F6- Guide for Secondary Containment of Hazardous Gas Piping Systems

• SEMI F13- Guide for Gas Source Control Equipment

• SEMI F14- Guide for Design of Gas Source Equipment Enclosures

• SEMI S2- Environmental, Health, and Safety Guideline for Semiconductor Manufacturing Equipment

• SEMI S4- Safety Guideline for the Segregation/Separation of Gas Cylinders Contained in Cabinets

• SEMI S6- EHS Guideline for Exhaust Ventilation of Semiconductor Manufacturing Equipment

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Gas Cabinets and Gas Boxes

Gas boxes are a typical 1/4-inch delivery system, with assumed that regulated gas of no greater than 50 psi will be used in furnaces, chemical vapor deposition (CVD) tools, and etch reactors. Systems are designed for flows no greater than 30 slm.

Cylinder storage problems are simplified because the use of gas cabinets encourages separation of gases according to their hazard classification. Separation of gases becomes standard procedure, as opposed to indiscriminate storage and grouping. For example, corrosives, oxidizers, flammables and toxics can be separated and grouped into separate cabinets. Not only will this satisfy both the national and local fire and building codes, but it is also safer in the event of component failure or leakage.

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Gas Cabinet Objectives

A properly designed gas cabinet system will fulfill the following objectives:

• To control air flow around compressed gas cylinders and manifolds - isolating hazardous gases from operators

• To minimize hazards from compressed gases in event of a fire external or internal to the gas cabinet

• Maintenance of gas integrity

• Automatic shutoff of gas in the event of catastrophic failure

• Effective control of residual gas during cylinder changeout

A gas cabinet with

panel-mounted

manifolds.

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Materials of Construction

Use of 12 gauge steel or thicker for the cabinet and door will ensure sturdiness and also provide a half-hour or more of fire protection.

If poisonous gases are to be kept in the cabinet, an access port or window must be provided so the cylinder valves can be closed and leaks detected without opening the cabinet door and compromising the exhaust system.

Must have self-closing doors, self-closing limited access ports or fire rated windows.

Gas cabinets, cylinders and delivery lines must be seismic braced.

Provide welded connections or exhausted enclosures for all Class I & II gases.

For corrosive gases, primary piping shall be constructed of inert materials, or secondary containment shall be provided.

Provide an approved manual activation control for all Class I & II gases.

Material compatibility is an important consideration in selecting equipment for gas applications. An improper selection can result in problems ranging from corrosion-caused contaminants to catastrophic failure. It is recommended that users be aware of any potential hazard before using a gas and take the proper precautions.

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Ventilation

In order to provide containment of potentially dangerous gases, cabinet exhaust systems should be designed with the capability to allow an average of 200 linear feet per minute (61 linear meters per minute) of air to pass through the cabinet with the access window open with a minimum single point of 150 linear feet per minute (45.7 linear meters per minute). This is roughly equivalent to 13 air changes per minute. A ventilated gas cabinet facilitates the safe management of the gases stored within it.

To ensure ventilation is working as specified, a photohelic gauge is used to monitor static pressure in the duct.

Gas cabinets must be connected to an abatement (treatment) system.

No exterior storage area shall be within 75 feet of a fresh air intake.

Cabinets shall be constructed to sweep the exhaust by the cylinder connection and gas manifold, preventing pocketing of gases or dead spaces.

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Fire Protection

As an extra measure of fire protection, all gas cabinets shall be equipped with an integral sprinkler system. While exact requirements may vary with the specific application, a typical sprinkler would have a fusible link rated at about 135°F (57°C) and a flow capability of approximately 40 GPM (2.524 L/s). If used with corrosive gases, they must be coated for protection from corrosion.

“No smoking” signs must be posted within 25 feet of outdoor storage, use, and handling areas.

Provide a local manual emergency alarm, emergency telephone, or signaling device where Class I & II gases are transported through exit corridors or exit enclosures.

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Chemical Protection

Gas cabinets with toxic gases shall have a continuous gas-detection system to monitor for potential leaks. If a leak is detected, it should initiate local and area audible and visual alarms and automatically shut down the delivery system.

Monitor the room or area where the gas is stored at or below the Permissible Exposure Limit (PEL)

Monitor the discharge from a treatment system to ensure it is at or below Immediately Dangerous to Life or Health (IDLH)

A special leaker gas cabinet should be provided so a leaking cylinder can be put into this cabinet. The leaker cabinet should have a higher exhaust flow rate.

The number of cylinders contained in a single gas cabinet shall not exceed three, one of which must be an inert purge gas.

Pressure relief valves must go to the abatement system.

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Gas Detection

Choosing the correct gas detection system for your application takes skill and understanding of the hazards, potential interferences, and conditions of the area to be monitored.

Toxic gas detectors may be electrochemical, metal oxide semiconductor (MOS), chemically impregnated paper tape, photoionization, pyrolysis, galvanic cell, catalytic combustion, and infrared sensing elements.

They can be in the exhaust stream, inside the gas cabinet, or sample draw type to prevent dust and dirt from affecting the sensors.

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Some Advantages/Disadvantages

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Most Important

Calibration with a zero air standard and a certified calibration standard concentration of the gas being monitored.

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Automatic Shutdown for All Class 1 Gases

Provide an automatic "fail-safe to close" shut-off valve for the following: 1. Gas detection 2. Remote location alarm 3. Failure of emergency power 4. Seismic activity 5. Failure of primary containment 6. Activation of manual fire alarm

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Restricted Flow Orifice and Treatment Systems

Restricted Flow Orifice (RFO). All gas cylinders with TGO Class I gases (except for lecture bottles) having a vapor pressure greater than 29 psia shall have a restricted flow orifice.

All cylinders shall be marked with RFO size.

Treatment system must be capable of reducing the maximum allowable discharge concentration of the gas to one-half the IDLH at the point of discharge.

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Maintenance

Annual maintenance is required for all safety control systems. Ex: Monitoring system, scrubbers, seismic detectors, ventilation, etc.

Records must be maintained to prove adequate maintenance of all safety control systems.

The ventilation rate of every mechanical ventilation system used to prevent harmful exposure shall be tested after initial installation, alterations, or maintenance, and at least annually, by means of a pitot traverse of the exhaust duct or equivalent measurements. Records of these tests shall be retained for at least five years (Cal/OSHA Title 8, §5143. General Requirements of Mechanical Ventilation Systems).

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Emergency Response

An emergency response plan must be developed and submitted to the AHJ.

Submit or update the emergency response plan component of the Hazardous Materials Business Plan (HMBP),

Quarterly emergency drills must be conducted and maintain records of same for three years.

Gas alarms must transmit an alarm signal to a constantly attended control station for two or more cylinders.

Provide a minimum of two breathing apparatus and other appropriate protective equipment for Class I or corrosive gases. Ensure they are placed nearby, but in a safe location to put them on.

Must provide emergency power for the following: 1. Exhaust ventilation and treatment system 2. Gas detection system 3. Emergency alarm system 4. Temperature control systems

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Permits and Regulated Gas Releases

Permits must be submitted for storing, use, or handling of new regulated gases, new gas cabinets, relocations, and decommission and closures.

Submit report of incident and immediate notification of the fire department for any unauthorized releases.

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TGO Gases

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TGO Gases

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TGO Gases

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TGO Gases

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Valves

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Valve Specs

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Regulators

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Regulator Specs

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MFC/MFMs

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MFC/MFM Specs

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Pressure Transducers

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Pressure Transducer Specs

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Filters/Purifiers

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Filters/Purifiers Specs

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Resources

SEMATECH Design Guidelines for Gas Box Components, Technology Transfer 96063137A-ENG

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Best Engineering Practices

• Best Engineering Practices

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Photohelic Gauge vs Magnahelic Gauge

Gas cabinets must be equipped with an exhaust failure monitor alarmed and interlocked to shut off the gas by the cylinder.

Both gauges are indicators of static pressure relative to the amount of exhaust flow in the duct.

• A photohelic has two contacts for shutdown and alarm controls (orange bars) which are set by the customer for upper and lower safe operating setpoints upon system exhaust certification.

• A magnahelic has no contacts and is used as an indicator only.

• Various models are available and the proper scale must be specified for each use.

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Safety Controls are not "Customer Options"

• Many manufacturers today are under great cost pressures when dealing with customers in Asia. Many have requested stripped-down versions of the standard product offerings with safety built-in.

This is not like ordering a new car with a radio-delete. More like ordering a car without brakes and no air bags.

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EHS considerations when buying used equipment

• Do you know for sure what the prior gases used were and are they compatible with what gases you want to use

• Pump purge is not sufficient to run incompatible gases

• Was the gas box/cabinet properly decommissioned?

• Are there any parts missing?

• Do the automated controls work properly? Pump purge? EMO?

• Is there evidence of prior “dusting” (silane) or corrosion from past leaks (HBr)?

• Do the doors still close automatically?

• Any physical damage to the gas box/cabinet?

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Exhaust considerations

For best exhaust flow, use the largest duct diameter possible, eliminate as many 90 degree turns as possible, use 45 degree “Y” connections for lowest loss of static pressure, don’t use square duct from HVAC folks.

Ensure that the fan is placed as close to the outlet of exhaust as possible as the duct will be positive pressure after going through the fan and leaks before the fan will be into the duct and after the fan will be out of the duct. Avoid positive pressure ducts in occupied areas.

Ensure that there is sufficient flow to prevent fallout of contaminants prior to abatement.

Ensure exhausted materials are compatible on the way to the abatement device.

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Exhaust considerations

All non-welded connections for toxics and flammables must be within the exhausted gas cabinet. That means purifiers, MFCs, etc.

If duct depositions are possible, plan for removable sections for ease of cleaning with periodic preventive maintenance.

Do not presume prior tracer gas tests meet today’s requirements- example the Arsine Threshold Limit Value- Occupational Exposure Limit changed in 2007 from 50 ppb to 5 ppb. Under SEMI S6 requirements tracer gas testing must show leakage from the enclosure to be less than 1.25 ppb from the worst case release scenario.

Safe Delivery Systems (SDS) while under vacuum in normal use may not be in a fire situation. Many jurisdictions are adding additional requirements.